1. Field of the Invention
The present invention relates to an IC tag, an IC tag system, and a method of executing a command of the IC tag. In particular, the invention relates to an IC tag and an IC tag system that execute a specific command with key data and the specific command, and to a method of executing a command of the IC tag.
2. Description of Related Art
In recent years, attentions have been paid to a technique regarding RFID (Radio Frequency IDentification) in the field of physical distribution management in a factory or retail shop. This technique aims at attaching a tag including an IC storing specific information on a product to the product and scanning the information with a wireless antenna.
This technique uses a reader/writer and an RFID tag (hereinafter referred to as IC tag). The reader/writer sends a command with a modulated radio signal including data and a carrier to an IC tag, and receives a radio signal from the IC tag. The IC tag demodulates the received radio signal to execute processing in accordance with the received command. If requested to send response data with respect to the received command, the IC tag sends the response data to a reader/writer. In this example, the IC tag is prepared by integrating an IC chip and an antenna. The command is a signal transmitted with a low-frequency signal superimposed on a carrier of a radio signal.
Among the IC tags, a so-called passive type IC tag receives a radio signal from a reader/writer, and a rectifier in the IC tag generates a power supply voltage using a high-frequency carrier of the radio signal (for example, signal having a frequency of 2.45 GHz). That is, in the passive type IC tag, the radio signal for data communication with the reader/writer is utilized for power supply and data transmission/reception.
A technique of generating a power supply voltage in the circuit to drive the circuit with a received radio signal as described above is reported by Udo Kartbaus et al. in “Fully Integrated Passive UHF RFID Transponder IC With 16.7-μW Minimum RF Input Power”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 38, NO. 10, October 2003, p. 1602-1608. FIG. 6 is a block diagram of a conventional IC tag 100 based on this technique.
The IC tag 100 of FIG. 6 receives a radio signal through an antenna 200. A rectifier 311 of an IC chip 300 generates a power supply voltage based on a carrier of the received radio signal. The generated power supply voltage is utilized as a power supply voltage for driving each block of the IC chip 300.
Further, a detector 312 of the IC chip 300 generates a pulse signal based on the received radio signal. A command control circuit 302 generates a command based on the generated pulse signal to control a transmitter 303 or a memory control circuit 304, or to transmit write data and a write address. In response to the signal from the command control circuit 302, the memory control circuit 304 controls an operation of writing/reading data to/from a storage circuit 305. In this example, the data writing operation is to write the write data sent from the command control circuit 302 into a memory cell of the storage circuit 305 corresponding to address data. In addition, the data reading operation is to read the read data stored in a memory cell of the storage circuit 305 corresponding to address data from the command control circuit 302 and input the read data into an input register 305 of the memory control circuit 304. The read data input to the input register 305 is transferred to a transmitting register 345 under the control of the command control circuit 302 and then sent to a reader/writer (not shown) through the transmitter 303 and the antenna 200.
Here, description is given of a radio signal transmitted/received between the reader/writer and the IC tag 100. FIG. 7A shows a pulse signal generated from a radio signal with a detector 312, and FIG. 7B shows the radio signal of FIG. 7A. As shown in FIG. 7A, the pulse signal represents “no data”, “data 0”, and “data 1” in accordance with the number of pulses in a frame of a predetermined period (unit time) from a falling edge of one frame pulse to a falling edge of the next frame pulse; the frame pulses are transmitted at regular intervals. The “no data” indicates a period in which only a frame pulse is transmitted, and the IC tag generates a clock signal used in an inner circuit thereof based on the frame pulse. The “data 0” corresponds a data signal of “0” and is expressed as a frame pulse and one pulse. The “data 1” corresponds to a digital signal of “1”, and is expressed as a frame pulse and two pulses. The frame pulse and the pulse are recognized as a pulse by the command control circuit 302 if its level is equal to or higher than a predetermined threshold level, and a pulse the level of which is lower than the threshold level is not recognized as a pulse by the command control circuit 302.
The frame pulse or signal representing “data 0” or “data 1” is sent from the reader/writer to the IC tag by way of a carrier as shown in FIG. 7B. As shown in FIG. 7B, the carrier is a signal of a predetermined amplitude of, for example, 2.45 GHz, and during a period where the carrier transmission is stopped, the above frame pulse or pulse representing data appears. Such a signal modulating system is called “ASK (Amplitude Shift Keying) modulation”.
The rectifier 311 of the IC tag 300 generates a power supply voltage from the carrier. Further, the detector 312 detects a smaller-amplitude portion of the carrier to generate a pulse. At this time, an amplitude of the pulse generated with the detector 312 is determined based on a level of the power supply voltage generated with the rectifier 311.
However, in certain use environments of the IC tag 100, a disturbance occurs in a radio signal due to influences of, for example, absorption of ambient moisture to the IC tag 100, a reflection from metal around the IC tag 100, and an interference of the antennal of the IC tag.
If the disturbance occurs in the radio signal, for example, a voltage generator cannot generate enough power supply voltage, resulting in a problem in that a detector erroneously operates to generate a wrong signal, not a pulse signal generated from a received radio signal or a problem in that the radio signal from the reader/writer is changed due to a noise, and the detector generates a wrong pulse signal, not a pulse signal to be sent from the reader/writer.
An example of the problems resulting from the generation of the wrong pulse signal is given below. In this case, if the read command is defined as “1011”, and an erase all command for information in the storage circuit 305 is defined as “1010”, for example, the above data recognition error occurs in the last bit of the read command, the read command is changed from “1011” to “1010” and becomes identical to the erase all command. As a result, there arises a problem in that all information in the storage circuit 305 is deleted although data is read from the storage circuit 305 under normal circumstances.